The present invention relates generally to the use of Code Division Multiple Access (CDMA) communication techniques in a radio communication system and, more particularly, to receivers which demodulate CDMA signals.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry""s growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.
Throughout the world, one important step in the advancement of radio communication systems is the change from analog to digital transmission. Equally significant is the choice of an effective digital transmission scheme for implementing next generation technology. Furthermore, it is widely believed that the first generation of Personal Communication Networks (PCNs), employing low cost, pocket-sized, cordless telephones that can be carried comfortably and used to make or receive calls in the home, office, street, car, etc., will be provided by, for example, cellular carriers using the next generation digital cellular system infrastructure. An important feature desired in these new systems is increased traffic capacity.
Currently, channel access is achieved using Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) methods. In FDMA, a communication channel is a single radio frequency band into which a signal""s transmission power is concentrated. Signals which can interfere with a communication channel include those transmitted on adjacent channels (adjacent channel interference) and those transmitted on the same channel in other cells (co-channel interference). Interference with adjacent channels is limited by the use of band pass filters which only pass signal energy within the specified frequency band. Co-channel interference is reduced to tolerable levels by restricting channel re-use by providing a minimum separation distance between cells in which the same frequency channel is used. Thus, with each channel being assigned a different frequency, system capacity is limited by the available frequencies as well as by limitations imposed by channel reuse.
In TDMA systems, a channel consists of, for example, a time slot in a periodic train of time intervals over the same frequency. Each period of time slots is called a frame. A given signal""s energy is confined to one of these time slots. Adjacent channel interference is limited by the use of a time gate or other synchronization element that only passes signal energy received at the proper time. Thus, with each channel being assigned a different time slot, system capacity is limited by the available time slots as well as by limitations imposed by channel reuse as described above with respect to FDMA.
With FDMA and TDMA systems (as well as hybrid FDMA/TDMA systems), one goal of system designers is to ensure that two potentially interfering signals do not occupy the same frequency at the same time. In contrast, Code Division Multiple Access (CDMA) is a channel access technique which allows signals to overlap in both time and frequency. CDMA is a type of spread spectrum communications, which has been around since the days of World War II. Early applications were predominantly military oriented. However, today there has been an increasing interest in using spread spectrum systems in commercial applications since spread spectrum communications provide robustness against interference, which allows for multiple signals to occupy the same bandwidth at the same time. Examples of such commercial applications include digital cellular radio, land mobile radio, and indoor and outdoor personal communication networks.
In a CDMA system, each signal is transmitted using spread spectrum techniques. In principle, the informational data stream to be transmitted is impressed upon a much higher rate data stream known as a signature sequence. Typically, the signature sequence data are binary, providing a bit stream. One way to generate this signature sequence is with a pseudo-noise (PN) process that appears random, but can be replicated by an authorized receiver. The informational data stream and the high bit rate signature sequence stream are combined by multiplying the two bit streams together, assuming the binary values of the two bit streams are represented by +1 or xe2x88x921. This combination of the higher bit rate signal with the lower bit rate data stream is called spreading the informational data stream signal. Each informational data stream or channel is allocated a unique signature sequence.
A plurality of spread information signals modulate a radio frequency carrier, for example by binary phase shift keying (BPSK), and are jointly received as a composite signal at the receiver. Each of the spread signals overlaps all of the other spread signals, as well as noise-related signals, in both frequency and time. If the receiver is authorized, then the composite signal is correlated with one of the unique signature sequences, and the corresponding information signal can be isolated and despread. If quadrature phase shift keying (QPSK) modulation is used, then the signature sequence may consist of complex numbers (having real and imaginary parts), where the real and imaginary parts are used to modulate two carriers at the same frequency, but ninety degrees different in phase.
Traditionally, a signature sequence is used to represent one bit of information. Receiving the transmitted sequence or its complement indicates whether the information bit is a +1 or xe2x88x921, sometimes denoted xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d. The signature sequence usually comprises N bits, and each bit of the signature sequence is called a xe2x80x9cchipxe2x80x9d. The entire N-chip sequence, or its complement, is referred to as a transmitted symbol. The conventional receiver, e.g., a RAKE receiver, correlates the received signal with the complex conjugate of the known signature sequence to produce a correlation value. After compensation for linear distortion, only the real part of the correlation value is computed. When a large positive correlation results, a xe2x80x9c0xe2x80x9d is detected; when a large negative correlation results, a xe2x80x9c1xe2x80x9d is detected.
The xe2x80x9cinformation bitsxe2x80x9d referred to above can also be coded bits, where the code used is a block or convolutional code. Also, the signature sequence can be much longer than a single transmitted symbol, in which case a subsequence of the signature sequence is used to spread the information bit. In many radio communication systems, the received signal includes two components: an I (in-phase) component and a Q (quadrature) component. In a typical receiver using digital signal processing, the received I and Q component signals are sampled and stored at least every Tc seconds, where Tc is the duration of a chip.
FIG. 1 illustrates the conventional RAKE receiver. The conventional RAKE receiver 100 includes a multipath delay searcher 110, a plurality of parallel demodulators (commonly referred to in the art as RAKE xe2x80x9cfingersxe2x80x9d) 120 and a combiner 130. In general, the RAKE receiver exploits the multipath time delays in a channel and combines delayed replicas of a transmitted signal in order to improve link quality. The RAKE receiver captures most of the received signal energy by allocating a number of parallel demodulators 120 to the selected strongest components of the received multipath signal which are determined by the multipath delay searcher 110. One skilled in the art will appreciate that the multipath delay search processor (commonly referred to in the art as the xe2x80x9csearcherxe2x80x9d) 110 estimates the channel delay profile, identifies paths within the delay profile, and identifies the delay variations due to changing propagation conditions. After the corresponding delay compensation by the RAKE fingers 120, the outputs of all fingers are combined by combiner 130 in order to determine the information content of the signal.
One skilled in the art will appreciate that the searcher of the conventional RAKE receiver consumes a significant portion of the receiver""s total power expenditure. Therefore, in order to prolong the battery life of a mobile station into which the RAKE receiver is implemented, it is important to keep the duty-cycle of the searcher as low as possible.
The conventional RAKE receiver fails to consider how quickly the mobile station""s environment is changing when performing the searching and demodulation processes. As a result, the duty cycle of the searcher remains constant irrespective of changing environmental conditions, thereby needlessly wasting valuable power.
The present invention recognizes that, by considering the rate of change of the mobile station""s environment, the duty cycle of the searcher can be optimized so as to save power. In addition, channel tracking can be improved.
The present invention seeks to reduce the power consumption of conventional RAKE receivers. In exemplary embodiments of the present invention, the rate of change of the mobile station""s environment (reflected in path variations) is determined and routed to the searcher of the RAKE receiver. When, for example, the mobile station""s environment is changing quickly, the duty cycle of the searcher is modified so that the search operation is performed more frequently. On the other hand, a slowly changing environment allows for the search operation to be performed less frequently thereby reducing the power consumed by the mobile station.
According to an embodiment of the present invention, a motion detector is implemented in the CDMA mobile station. The motion detector is connected to the searcher and provides an estimate (e.g., very low, low, medium, etc.) of the velocity of the mobile station. In this case velocity of the mobile station can be used as a proxy for the changing environment of the mobile, e.g., as the velocity of the mobile station increases, so does the rate of path variation. The velocity estimate thus allows the duty cycle of the searcher to be optimized, thereby reducing the overall power consumed by the receiver. By also providing the motion estimate to the RAKE fingers, channel estimation tracking is improved.